Research On Technologies In Ofdm Systems For Large-Scaled Wireless Seismic

In Large-scale and high-resolution seismic exploration,the concurrent seismic data from the thousands of near-placed nodes should be collected as soon as possible via cable-free communication systems,which require high-speed and reliable physical layer technology.Orthogonal frequency division multiplexing(OFDM)technology features high spectrum utilization,simple balance,and strong anti-interference and anti-multipath abilities,and so is very suitable for field data transmission in wireless seismic exploration.OFDM-IM(OFDM with Index Modulation)is a novel OFDM transmission scheme,which extends the principle of spatial modulation to OFDM subcarriers.It can carry additional bits of information by using the indices combination of active subcarriers(Called Index modulation)and effectively improve bit error ratio(BER)performance of OFDM system under multipath channels.In order to improve the reliability of OFDM system in wireless seismic exploration,this dissertation focuses on the key technologies on OFDM-IM technology and its synchronization issues.The main innovative technologies are proposed in the following aspects:(1)A low-complexity maximum likelihood(LC-ML)detector and a low-complexity near maximum likelihood(LC-NML)detector were proposed for the symbol detection at the receiver of OFDM-IM system for the small and large p1 bits in index modulation(IM)respectively.The LC-ML and the improved LC-NML can greatly reduce the complexity of the traditional ML detection but almost have the same BER performance as the traditional ML detector.(2)A new dual-mode OFDM-IM scheme termed dual-mode index modulation aided OFDM with constellation power allocation(DM-OFDM-CPA)was proposed to make full use of the 'inactive' subcarriers in OFDM-IM.In DM-OFDM-CPA,the 'inactive' subcarriers of OFDM-IM are used to transmit modulated symbols and the subcarriers in each subblock are divided into two groups,which are set different power levels and modulated by different M-ary PSK symbols for IM.The simulation results under the HT multipath channel showed that the proposed DM-OFDM-CPA not only ensured a significantly better BER performance but also eliminated the limitation of spectrum efficiency(SE).(3)An enhanced permutation method(EPM),which has an equal subcarrier modulation probability for a given mode,and a corresponding optimal bit mapping rule were proposed to improve the IM performance for multi-mode OFDM-IM(MM-OFDM-IM),which also makes full use of the 'inactive' subcarriers.The simulation results under the HT multipath channel showed that the optimal EPM scheme had a better BER performance for IM and the signal-to-noise ratio gain achieved 1.4dB.(4)An improved Park's algorithm applying a scrambling operation to the first half sequence of the training symbol was proposed to mitigate the influence of NBI on time-frequency synchronization for OFDM system in wireless seismic exploration.The proposed algorithm not only ensures that the timing metric based on the conjugate-symmetric auto-correlation operation still maintains the sharp peak property under NBI,but also mitigates the influence of NBI on the carrier frequency offset(CFO)estimation based on repeat auto-correlation operation.The simulation results under the RA multipath channel showed that the proposed algorithm still completed time-frequency synchronization quickly and accurately when the signal-to-interference ratio equals to 0.In addition,an improved time-frequency synchronization algorithm by the design of a multi-repetitive training symbol based on the above algorithm was proposed for the requirement of multiple CFO estimation.The improved algorithm obtains a larger estimation range of CFO in time domain while ensuring sharp timing peak and anti-NBI properties.The simulation results under the RA multipath channel showed that the improved algorithm obtained a good time-frequency synchronization performance under NBI when the ICFO(integer CFO)estimation range is-2?2 and-4?4.(5)A new training symbol Ps and a joint scheme for simultaneously implementing 8bits/10bits signaling transmission and precise time-frequency synchronization were proposed to satisfy the various multipath channels transmission in practical seismic exploration.The simulation results under the RA and HT multipath channels showed that the proposed scheme not only provided more precise symbol time offset and fractional CFO estimation and good anti-NBI property,but also obtained accurate large ICFO estimation and signaling transmission to meet the adaptive requirements of the transmission channels in field seismic exploration.